Dual axis geophones for pressure/velocity sensing streamers forming a triple component streamer

ABSTRACT

A section of a streamer for acoustic marine data collection, the section comprising a carrier for accommodating seismic sensors, wherein the carrier includes, a single body, a first particle motion sensor located on the single body, and a second particle motion sensor being located on the single body, with a 90° angular offset, about a longitudinal axis of the carrier, relative to the first particle motion sensor; and a tilt sensor coupled to the carrier and having a known direction relative to the first and second particle motion sensors so that the tilt sensor determines an angle of tilt of the carrier about a vertical, wherein the first and second particle motion sensors measure a motion related parameter and not a pressure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation application which claims priorityunder 35 U.S.C. 120 to U.S. application Ser. No. 13/664,708, filed onOct. 31, 2012, which is a Continuation Application of U.S. applicationSer. No. 13/493,361, filed on Jun. 11, 2012 which is a ContinuationApplication of U.S. application Ser. No. 13/161,896, filed on Jun. 16,2011 which claims priority to U.S. Patent Application No. 61/356,835filed on Jun. 21, 2010, all of which are hereby expressly incorporatedby reference into the present application.

FIELD OF THE INVENTION

The present invention relates generally to the field of seismicstreamers which are towed through water behind vessels for seismicexploration, and, more particularly, to the field of non-fluid filledseismic streamers. Even more particularly, the present invention relatesto a seismic streamer which includes a pair of orthogonal acousticparticle motion sensors, such as geophones or accelerometers, in thesame segment or in close proximity to a plurality of pressure sensors,such as hydrophones.

BACKGROUND OF THE INVENTION

The background in related art is described by Vaage et al. in U.S. Pat.No. 7,684,281. In seismic exploration, geophysical data are obtained byapplying acoustic energy to the earth from an acoustic source anddetecting seismic energy reflected from interfaces between differentlayers in subsurface formations. The seismic wavefield is reflected whenthere is a difference in acoustic impedance between the layers on eitherside of the interface. Typically in marine seismic exploration, aseismic streamer is towed behind an exploration vessel at a water depthnormally between about six to about nine meters, but can be towedshallower or deeper. Hydrophones are included in the streamer cable fordetecting seismic signals. A hydrophone is a submersible pressure sensorthat converts pressure waves into electrical or optical signals that aretypically recorded for signal processing, and evaluated to estimatecharacteristics of the subsurface of the earth.

In a typical geophysical exploration configuration, a plurality ofstreamer cables are towed behind a vessel. One or more seismic sourcesare also normally towed behind the vessel. The seismic source, which isoften an air gun array, but may also be a water gun array or other typeof source known to those of skill in the seismic art, transmits seismicenergy or waves into the earth and the waves are reflected back byinterfaces in the earth and recorded by sensors in the streamers. Wingedhydrodynamic actuators are typically employed to maintain the cables inthe desired lateral position while being towed. Alternatively, theseismic cables are maintained at a substantially stationary position ina body of water, either floating at a selected depth or lying on thebottom of the body of water, in which case the source may be towedbehind a vessel to generate acoustic energy at varying locations, or thesource may also be maintained in a stationary position.

When the reflected wave reaches the streamer cable, the wave is detectedby the hydrophones in the streamer cable as the primary signal. Thereflected wave also continues to propagate to the water/air interface atthe water surface, from which the wave is reflected downwardly, and isagain detected by the hydrophones in the streamer cable. The watersurface is a good reflector and the reflection coefficient at the watersurface is nearly unity in magnitude and is negative in sign forpressure signals. The waves reflected at the surface will thus bephase-shifted 180° relative to the upwardly propagating waves. Thedownwardly propagating wave recorded by the receivers is commonlyreferred to as the surface reflection or the “ghost” signal. Because ofthe surface reflection, the water surface acts like a filter, whichcreates spectral notches in the recorded signal, making it difficult torecord data outside a selected bandwidth. Because of the influence ofthe surface reflection, some frequencies in the recorded signal areamplified (constructive interference) and some frequencies areattenuated (destructive interference).

Maximum attenuation will occur at frequencies for which the propagationdistance between the detecting hydrophone and the water surface is aninteger multiple of one-half wavelength. Maximum amplification willoccur at frequencies for which the propagation distance between thedetecting hydrophone and the water surface is an integer multiple ofone-quarter wavelength. The wavelength of the acoustic wave is equal tothe velocity divided by the frequency, and the velocity of an acousticwave in water is about 1500 meters/second. Accordingly, the location inthe frequency spectrum of the resulting first (lowest-frequency)spectral notch can be readily determined. For example, for a seismicstreamer at a depth of 7 meters, and waves with vertical incidence,maximum attenuation will occur at a frequency of about 107 Hz andmaximum amplification will occur at a frequency of about 54 Hz.

It has not been common practice to tow streamer cables deeper than aboutnine meters because the location of the lowest-frequency spectral notchin the frequency spectrum of the signal detected by a hydrophonesubstantially diminishes the utility of the recorded data. It has alsonot been common practice to tow streamer cables at depth less than sixmeters, because of the significant increase in surface related noiseinduced in the seismic streamer data.

It is also common to perform marine seismic operations in which sensorsare deployed at the water bottom. Such operations are typically referredto as “ocean bottom seismic” operations. In ocean bottom seismicoperations, both pressure sensors (hydrophones) and particle motionsensors (geophones, accelerometers) are deployed at the ocean floor torecord seismic data.

A particle motion sensor, such as a geophone, has directionalsensitivity, whereas a pressure sensor, such as hydrophone, does not.Accordingly, the upgoing wavefield signals detected by a geophone andhydrophone located close together will be in phase, while the downgoingwavefield signals will be recorded 180° out of phase if the geophone isoriented in a particular direction. Various techniques have beenproposed for using this phase difference to reduce the spectral notchescaused by the surface reflection and, if the recordings are made on theseafloor, to attenuate water borne multiples. It should be noted that analternative to having the geophone and hydrophone co-located, is to havesufficient spatial density of sensors so that the respective wavefieldsrecorded by the hydrophone and the geophone can be reconstructed(interpolated) at a convenient location in the vicinity of the spatialdistribution of sensors.

U.S. Pat. No. 4,486,865 to Ruehle, for example, teaches a system forsuppressing ghost reflections by combining the outputs of pressure andvelocity detectors. The detectors are paired, one pressure detector andone velocity detector in each pair. A filter is said to change thefrequency content of at least one of the detectors so that the ghostreflections cancel when the outputs are combined.

U.S. Pat. No. 5,621,700 to Moldovenu also teaches using at least onesensor pair comprising a pressure sensor and a velocity sensor in anocean bottom cable in a method for attenuating ghosts and water layerreverberations.

U.S. Pat. No. 4,935,903 to Sanders et al. teaches a marine seismicreflection prospecting system that detects seismic waves traveling inwater by pressure sensor-particle velocity sensor pairs (e.g.,hydrophone-geophone pairs) or alternately, vertically-spaced pressuresensors. Instead of filtering to eliminate ghost reflection data, thesystem calls for enhancing primary reflection data for use in pre-stackprocessing by adding ghost data.

U.S. Pat. No. 4,979,150 to Barr provides a method for marine seismicprospecting said to attenuate coherent noise resulting from water columnreverberation by applying a scale factor to the output of a pressuretransducer and a particle velocity transducer positioned substantiallyadjacent to one another in the water. Barr states that the transducersmay be positioned either on the ocean bottom or at a location in thewater above the bottom, although the ocean bottom is said to bepreferred.

U.S. Pat. No. 7,239,577, to Tenghamn describes a particle motion sensorfor use in a streamer cable and a method for equalizing and combiningthe output signals of the particle motion sensor and a co-locatedpressure gradient sensor.

As the cited patents show, it is well known in the art that pressure andparticle motion signals can be combined to derive both the up-going andthe down-going wavefield. For sea floor recordings, the up-going anddown-going wavefields may subsequently be combined to remove the effectof the surface reflection and to attenuate water borne multiples in theseismic signal. For towed streamer applications, however, the particlemotion signal has been regarded as having limited utility because of thehigh noise level in the particle motion signal. However, if particlemotion signals could be provided for towed streamer acquisition, theeffect of the surface reflection could be removed from the data.

U.S. Pat. No. 7,123,543 describes a procedure for attenuating multiplesby combining up- and down-going wavefields, measured in the watercolumn, where the wavefields are calculated from combining pressuresensors like hydrophones and motion sensors like geophones. Theprocedure assumes, however, that both the pressure and the motion datahave the same bandwidth.

It has been difficult to achieve the same bandwidth in the motion sensordata as in the pressure sensor data, however, because of the noiseinduced by vibrations in the streamer, which is sensed by the particlemotion sensors. The noise is, however, mainly confined to lowerfrequencies. One way to reduce the noise is to have several sensors inseries or in parallel. This approach, however, does not always reducethe noise enough to yield a signal-to-noise ratio satisfactory forfurther seismic processing.

A combination of acoustic pressure and particle velocity can inprinciple be used to discriminate the direction of acoustic wavefront.This technique has a long history in the world of ‘velocity’microphones.

In the field of marine geophysics, acoustic particle velocity sensing isoften done with geophones (typically electrodynamic velocity sensors).The motion of a neutrally-buoyant cable is taken to be a good analog ofthe acoustic particle velocity, at least over some frequency range andsome angle of incidence range. To minimize cost and complexity somevendors use a single axis gimbaled sensor on the assumption that onlyvertically-oriented wavefronts are of primary interest.

Historically, vertically oriented wavefronts were in fact the primaryconcern, but in modern geophysics there is increased interest inwavefronts arriving from a broad range of angles, so the gimbaled singleaxis sensor is not optimal.

High quality gimbals are not inexpensive, and even the best introducethe possibility of slip ring noise, and it is common practice to addfluid damping so that the geophone sensor orientation may lag the actualcable orientation in the presence of roll. Such a lag would introduceerrors in the measured acoustic particle velocity.

In the case of ‘solid’ cable streamers such as Sercel's Sentinel®streamer, gimbals pose a very difficult problem in that the gimbaledsensor needs to have its center of gravity exactly on the cable centerof gravity, yet that space is occupied by stress member and electricalwiring. SENTINEL® is a registered trademark of Sercel, Inc. A pair oforthogonal particle motion sensors with their active axes passingthrough the cable axis bypass the geometrical problems as well as thepotential for lag and slip ring noise while also allowing for thepossibility of discrimination of other-than-vertical wavefront arrivals.

For orthogonal particle motion sensors, separate tilt (rotation) sensingmeans must be provided (e.g. accelerometers with adequate DC accuracy)in order to determine direction based on gravity. A pair of orthogonalhigh quality DC-responsive accelerometers could serve both for velocitysensing and directional sensing, but the embodiments described hereinuse less expensive components.

Every sensor poses a cost in terms of data acquisition bandwidth.Obviously the single gimbaled velocity sensor is a lowest-cost approach,but with local signal processing the dual axis particle motion sensorplus tilt sensing can be reduced to an equivalent bandwidth load if thefunctionality of off-vertical discrimination is sacrificed.

In conclusion, the dual orthogonal sensor approach resolves difficultdesign problems as well as providing much more valuable information tothe geophysicist in the way of velocity components.

Thus, a need exists for a method for obtaining a useful particle motionsignal with a satisfactory signal-to-noise ratio at low frequencies. Inparticular, a need exists for a method to generate a particle motionsignal with substantially the same bandwidth as a recorded pressuresignal, for particle motion and pressure sensors located in a towedmarine seismic streamer. Unfortunately, the proposed solutions thus fardescribed are far too complex and expensive to find wide application inthe field, and the complexity of these solutions leads to unacceptablyhigh failure rates in operation. In particular there exists a need for asimple, inexpensive structure to combine pressure and particle motionsignals in a marine seismic cable to eliminate or minimize ghosts. Theinvention disclosed herein is directed to fulfilling that need in theart.

SUMMARY OF THE INVENTION

The present invention addresses these and other shortcomings in the artof marine seismic streamers by providing a plurality of hydrophones anda plurality of pairs of orthogonally oriented geophones in a marineseismic cable. The cable also includes a tilt sensor adjacent or inclose proximity to the particle motion sensors to indicate the verticalorientation of the particle motion sensors. The combination of theparticle motion sensors and the tilt sensor develops a signal which is afunction of the direction of incidence, therefore indicating if a signalhas been reflected off the overlying air/water interface, and can thusbe filtered from the overall seismic signal.

These and other features, objects, and advantages of the presentinvention will be readily apparent to those of skill in the art from areview of the following detailed description along with the accompanyingdrawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features, advantages andobjects of the present invention are attained and can be understood indetail, more particular description of the invention, briefly summarizedabove, may be had by reference to embodiments thereof which areillustrated in the appended drawings.

FIG. 1 is an overall schematic of a marine seismic system wherein thepresent invention may find application.

FIG. 2 is a perspective view of a hydrophone carrier, adapted to carry apair of orthogonally oriented particle motion sensors (specificallyaccelerometers).

FIG. 3 is a section view of an accelerometer, from which particlevelocity can be extracted and which may find application in the presentinvention.

FIG. 4 is perspective view of a particle motion sensor carrier, adaptedto carry a pair of orthogonally oriented particle motion sensors andwhich may or may not include a plurality of hydrophones.

FIG. 5 is a section view of the carrier of FIG. 4.

FIG. 6 is a top view of the carrier of FIG. 4.

FIG. 7 is side section view of the carrier of FIG. 4.

FIG. 8 is a perspective view of a hydrophone carrier constructed inaccordance with the teachings of the present invention.

FIG. 9 is a longitudinal section view of the hydrophone carrier of FIG.8.

FIG. 10 is a radial section view of the hydrophone carrier of FIG. 8,taken along the section lines 10-10 of FIG. 9.

FIG. 11 is a side view of a streamer including a plurality ofhydrophones, accelerometers, tilt sensors, and electronics to convertanalog seismic signals to digital format for transmission back to avessel.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 depicts a schematic of a basic marine system including a vessel10 towing a streamer 12. The streamer has a number of pieces ofauxiliary equipment, such as depth control devices, associated with itthat are not shown in order to simplify FIG. 1.

The streamer 12 also includes a number of hydrophone carriers 14 spacedapart along the streamer. As used herein, the term “hydrophone” refersto the active elements which are sensitive to the seismic signals(acoustic pressure) and the supporting body (or structure) which retainsthe active elements is referred to as a “hydrophone carrier”. Activeelements typically comprise piezoelectric elements, but may also includeoptical elements, micro-machined electro-mechanical sensor elements, andthe like. In the present invention, the hydrophone carrier is adapted toretain not only the hydrophones but also a pair of orthogonally orientedgeophones and a tilt sensor.

The hydrophone carriers 14 and a buoyant material are sealed within anouter jacket 16, preferably made of polyurethane, to present a smoothprofile, thereby minimizing flow noise. During seismic operations, thestreamer 12 is deployed from a cable reel 18 and, once operations arecomplete, the streamer 12 is reeled back onto the cable reel 18.

As one example, a streamer 12 may be comprised of a plurality ofsections, each section 150 meters in length. Each section includestwelve hydrophone groups, with eight hydrophones per group. Between eachhydrophone group is a particle motion sensor (accelerometer) group, withfour accelerometers per group comprising two channels per hydrophonegroup. This arrangement is shown and described below in greater detailin respect of FIG. 11.

FIG. 2 illustrates a preferred hydrophone carrier 14. The carrier 14retains a plurality of hydrophones 20, arranged in opposing pairs inopposing wells 30. The carrier 14 also retains a top geophone 22 and aside geophone 24, which are arranged 90° about the cable longitudinalaxis relative to one another. The carrier also retains a tilt sensor 26to determine the angle of tilt of the carrier about its axis, relativeto vertical. In a preferred embodiment, four such mutually opposedgeophones 24 may be included. Further in a preferred embodiment, thetilt sensor may be deployed in a separate signal processing module, asdescribed below.

FIG. 3 illustrates an example of an accelerometer 22 or 24 which may beused in the application of the invention. The accelerometer 22 comprisesa piezoelectric element 32 mounted within a box 34 defining an interiorchamber 35. The box is secured to a base 36 which includes a hole 38through it. The hole 38 permits the inflow of fluid, preferably oil, asillustrated by an arrow 40. Without the hole 38, the accelerometer 22would instead behave as a hydrophone, generating an electrical signal inresponse to a pressure which would flex the box 34 and thus the element32. With the hole 38, pressure is equalized across the element 32, andthus the element 32 generates an electrical signal in response toacceleration of the device 22. The accelerometer 22 or 24 is mountedwithin the hydrophone carrier 14 through pliable grommets 39, preferablymade of rubber, in the same manner as shown and described in U.S. Pat.No. 7,382,689, assigned to the same Assignee as the present invention,and incorporated herein by reference. The grommets 39 help to isolatethe accelerometer 22 or 24 from vibrations created along the streamer.

FIGS. 4 through 7 illustrate a particle motion sensor carrier 50 of thisinvention. The carrier 50 include a top geophone 52 and a side geophone54 and a tilt sensor not shown in this embodiment. The carrier may alsoinclude a plurality of hydrophones, arranged in opposing pairs, butthese hydrophones are not shown in FIGS. 4 through 7 for simplicity.

The various elements are operatively mounted on a bulk cable 58, whichincludes strength members, power conductors, signal conductors, andfiller material. The geophone 52 is mounted within a molded carrier body60 which is in turn mounted to cable 58. A void 55 is provided formaking wiring connections. In-line vibration damping is provided byelement 57, which is adjacent to a molded isolator 56.

Finally, FIGS. 8-11 show an accelerometer section 150 of a presentlypreferred embodiment of this invention.

The invention comprises a streamer having a plurality of hydrophones, aspreviously described, aligned with a plurality of accelerometers whichdetect movement of the streamer in the horizontal and verticaldirections, all coupled with a tilt sensor, so that the marine seismicsystem can detect whether a detected seismic signal is a reflection froma geologic structure beneath the streamer or a downward travelingreflection from the air/seawater interface.

The accelerometer section 150 includes a top particle motion sensor 152and a bottom particle motion sensor 154, coupled together by a pair ofleads 156 to define a first signal channel. It also includes a rightparticle motion sensor 158 and a left particle motion sensor 160,coupled together by a pair of leads 162 to define a second signalchannel. Each of the particle motion sensors 152, 154, 158, and 160 arepreferably constructed as shown in FIG. 3. Thus, rather than a singlegeophone to detect vertical motion and a single geophone to detecthorizontal motion, as shown in FIGS. 2-7, the present preferredembodiment includes pairs of particle motion sensors additively coupledtogether to enhance signal-to-noise ratio.

As previously described, it is critical for proper operation of thisinvention that the orientation of the accelerometer section 150 and theadjacent accelerometer sections 150 that comprise the accelerometergroup be well known. Thus, the accelerometer section 150 includes a keyassembly 70. For purposes of description, the cable includes a forwardend 72 and an after end 74. The key assembly 70 at the forward end 72 isshown made up, while the key assembly 70 at the after end 74 is shown inan exploded view.

The streamer is covered by a jacket 76 in a manner well known in theart. In turn, the particle motion sensors 152, 154, 158, and 160 areenclosed within a sleeve 78. A notch 80 is formed in the sleeve to matewith a first key 82 formed in an end cap 84. A magnet 81 is positionedadjacent the first key 82. The magnet is used to find the orientation ofthe accelerometer after the final skin is extruded over the cable. It isnecessary to know the orientation of the accelerometers when calibratingthe offset between the tilt meter gravity measurement and theaccelerometer orientation. The end cap 84 also includes second key 86 tomate with a first notch 88 formed in a coupling member 90. The couplingmember further includes a second notch 92 which is arranged to mate withan adjacent section 94. This adjacent section 94 may be anotheraccelerometer section 150, a hydrophone group, or a field digitizingunit, as described below in respect of FIG. 11.

The accelerometer section 150 is shown in a side section view in FIG. 9.The top particle motion sensor 52 is mounted to the section 150 with abracket assembly 96 by, for example, screws 98, or other appropriatemeans. The sleeve 78 extends from the bracket assembly 96, and theentire section 150 is covered by the jacket 76 (see FIG. 8).

Finally, FIG. 11 shows a marine seismic streamer 100 assembled asdescribed herein in accordance with this invention. For descriptivepurposes, the left end of the streamer 100 is the forward end of thestreamer. The streamer is made up of a plurality of hydrophone carriers14 associated to a plurality of accelerometer sections 150. Betweengroups of hydrophone carriers and accelerometer sections is a fielddigitizing unit 102. The field digitizing unit 102 receives analogseismic signals from the hydrophone carriers 14 and converts theseanalog signals into digital form. The unit 102 also receives analogsignals from the accelerometer sections 150 through leads 56 and 62 andconverts these analog signals into digital form. Finally, the unit 102preferably includes a tilt sensor as previously described in respect ofFIG. 2.

The principles, preferred embodiment, and mode of operation of thepresent invention have been described in the foregoing specification.This invention is not to be construed as limited to the particular formsdisclosed, since these are regarded as illustrative rather thanrestrictive. Moreover, variations and changes may be made by thoseskilled in the art without departing from the spirit of the invention.

The invention claimed is:
 1. A section of a streamer for acoustic marinedata collection, the section comprising: a carrier for accommodatingseismic sensors, wherein the carrier includes, a single body, a firstparticle motion sensor located on the single body, and a second particlemotion sensor being located on the single body, with a 90° angularoffset, about a longitudinal axis of the carrier, relative to the firstparticle motion sensor; and a tilt sensor coupled to the carrier andhaving a known direction relative to the first and second particlemotion sensors so that the tilt sensor determines an angle of tilt ofthe carrier about a vertical, wherein the first and second particlemotion sensors measure a motion related parameter and not a pressure,and wherein each of the first and second particle motion sensorincludes, a base, a box mounted to the base, the box defining aninterior chamber, and the box having an interior upper surface, a holein the base, defining a fluid flow path into the interior chamber, and apiezoelectric element mounted on the interior upper surface of the box.2. The section of claim 1, wherein the first and second particle motionsensors are accelerometers.
 3. The section of claim 1, wherein the firstand second particle motion sensors are geophones.
 4. The section ofclaim 1, wherein the carrier further comprises: at least a hydrophone.5. The section of claim 1, further comprising: a jacket covering thecarrier; and a permanent magnet located inside the jacket for finding anorientation of the first and second particle motion sensors after afinal skin is extruded over the section.
 6. The section of claim 1,wherein the tilt sensor is located on the single body.
 7. The section ofclaim 1, further comprising: another carrier; and a field digitizingunit located between the carrier and the another carrier along thesection, wherein the field digitizing unit converts analog signals indigital signals.
 8. The section of claim 7, wherein the another carrierincludes particle motion sensors.
 9. The section of claim 7, wherein theanother carrier includes at least a hydrophone.
 10. The section of claim7, wherein the tilt sensor is located on the field digitizing unit. 11.A marine seismic cable for acoustic marine data collection, the cablecomprising: a plurality of hydrophones arranged in groups along thecable; at least one carrier that accommodates particle motion sensors,wherein the carrier includes, a single body, a first particle motionsensor located on the single body, and a second particle motion sensorbeing located on the single body, with a 90° angular offset, about alongitudinal axis of the carrier, relative to the first particle motionsensor; and a tilt sensor coupled to the carrier and having a knowndirection relative to the first and second particle motion sensors sothat the tilt sensor determines an angle of tilt of the carrier about avertical, wherein the first and second particle motion sensors measure amotion related parameter and not a pressure, and wherein each of thefirst and second particle motion sensor comprises: a base, a box mountedto the base, the box defining an interior chamber, and the box having aninterior upper surface, a hole in the base, defining a fluid flow pathinto the interior chamber, and a piezoelectric element mounted on theinterior upper surface of the box.
 12. The cable of claim 11, whereinthe first and second particle motion sensors are accelerometers.
 13. Thecable of claim 11, wherein the first and second particle motion sensorsare geophones.
 14. The cable of claim 11, wherein the carrier furthercomprises: at least a hydrophone.
 15. The cable of claim 11, furthercomprising: a jacket covering the carrier; and a permanent magnetlocated inside the jacket for finding an orientation of the first andsecond particle motion sensors after a final skin is extruded over thesection.
 16. The cable of claim 11, wherein the tilt sensor is attachedto the single body.
 17. The cable of claim 11, further comprising:another carrier; and a field digitizing unit located between the carrierand the another carrier along the section, wherein the field digitizingunit converts analog signals to digital signals.
 18. The cable of claim17, wherein the another carrier includes particle motion sensors. 19.The cable of claim 17, wherein the another carrier includes at least ahydrophone.
 20. The cable of claim 17, wherein the tilt sensor islocated on the field digitizing unit.